Echo ultrasound is an established technique in the area of medical imaging. Typically, an ultrasound imaging system has electronics for remote excitation of an ultrasound transducer array or probe to obtain cross-sectional images of the internal organs along a variety of planes.
The transducer array can be a linear array, a curved linear array or a phased array. The basic structure of each array includes a plurality of transducer elements which are arranged adjacent to one another along a surface. The sequence of exciting the transducers differs. In a linear array, the transducers are excited sequentially in a "tractor treading" pattern and form a rectangular image or "window". Typically, a multiplexor is used within the probe to effectuate the sequential excitation of the transducer elements. The multiplexor generates heat during the ultrasound procedure. The power used to excited the transducer elements also generates heat at the transducer array. A phased array system uses a non-sequential excitation of the transducer elements and the image resulting from a phased array transducer is typically a pie-shape. The phased array provides an image of the organ which may be blocked by dense tissue such as bone. For example, the phased array transducer produces an image which fans our from a point, typically the center of the array on the probe. The use of a phased array transducer enables a transthoracic probe to image the heart through various acoustic windows about the body. Heat is also generated at the transducer elements because of the power used to excite the elements.
Typically, the transmit frequency used in ultrasound imaging machines that generates satisfactory image clarity is in the range of 2-10 MHZ. When the aperture of the array is reduced, the frequency of the signal is typically increased (or wavelength reduced) to maintain the desired wavelength best suited to image an organ. However, when the frequency of the sound signal is increased, the signal becomes more quickly attenuated and the image penetration is reduced. One way to regain image penetration is to increase the power with which the signal is supplied to the transducer array. The increased power enables a smaller array to generate an image having the penetration of the larger lower frequency array. However, the increased power also generates an undesirable amount of heat at the transducer array. When using a transesophogeal imaging transducer, increased heat is especially problematic because the only place to dissipate the heat is within the esophagus. Since the temperature of a lens of a TEE probe cannot rise above 41.degree. C. per the FDA limit, the increased power, and the concomitant increase in temperature, may create an unsafe condition for the patient.
A desirable system is one that maintains the temperature of the transducer array below a predetermined limit which will not cause damage to body tissue, while providing adequate power to image small features at appropriate depths.